Crust hole repair for electrolytic cells

ABSTRACT

A process for repairing a hole in the crust of an electrolytic cell. The hole is repaired by covering it with a receptacle containing solid particles. 
     The receptacle comprises a polymeric material. More preferably, the receptacle comprises a cellulosic material, such as paper, polymer-impregnated paper, or cardboard. A closed paper bag having at least two paper layers and weighing about 15-20 lb. (6.8-9.1 kg) is particularly preferred. 
     When the electrolytic cell produces aluminum by electrolysis of alumina, the solid particles comprise an aluminum compound such as alumina, aluminum fluoride, cryolite, or a mixture of such compounds. Two preferred forms of alumina include smelting grade alumina (SGA) and alumina dust collected by an electrostatic precipitator (ESP dust).

FIELD OF THE INVENTION

The present invention relates to the repair of crust holes inelectrolytic cells for metal production. Such cells are commonly usedfor producing aluminum by electrolytic reduction of alumina dissolved ina molten electrolyte.

BACKGROUND OF THE INVENTION

Production of aluminum by electrolysis of alumina is a well-knownprocess. Commercial aluminum production is carried out in a reductioncell by the Hall-Heroult process in which alumina is dissolved in moltencryolite at about 960-980° C. An electric current passing through themolten electrolyte reduces alumina to aluminum which is collected in apool beneath the molten electrolyte bath.

Electric current enters the cell through an anode in contact with themolten electrolyte, passes downward through the electrolyte, through thepool of molten aluminum, and into a cathode which is formed integrallywith the cell bottom. The current leaves the cell through a metalcollector bar below the cell bottom and is conducted to an anode in thenext of a series of cells making up a pot line.

The cell operating temperature is maintained by resistance heating ofthe molten electrolyte, electrochemical reactions occurring in the cell,and insulating the cell structure. A frozen crust above the moltenelectrolyte helps to reduce heat loss because of its insulating effect.

However, the solid crust must be broken periodically to remove moltenmetal from the metal pool by a vacuum tap. This is achieved byperiodically breaking through the solid crust with a crust breakerapparatus. The broken crust collapses down into the molten electrolyteand melts, leaving a hole in the crust above the electrolyte. The crustis also broken periodically when carbon anodes are replaced.

Breaking the solid crust produces holes that increase heat loss from themolten electrolyte. For example, a hole in the crust having an area ofonly 1 square foot increases heat loss sufficiently that the cellvoltage must be increased by about 100 millivolts to maintain celltemperature.

Accordingly, there is a need for a means of repairing crust holes inorder to reduce heat loss from the molten electrolyte.

A principal objective of the present invention is to provide anefficient and economical process for repairing crust holes in anelectrolytic cell.

A related objective of the invention is to provide a crust hole repairprocess that avoids contaminating the electrolytic cell with substancesthat might interfere with cell operation or increase metal impuritylevels.

A further objective of the invention is to provide a crust hole repairprocess that does not pose health or safety risks to pot line workers.

An advantage of the invention is that it offers a means for recoveringalumina values in ESP dust collected at alumima refineries and aluminumsmelters.

Additional objectives and advantages of our invention will becomeapparent to persons skilled in the art from the following detaileddescription.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided anelectrolytic cell wherein a metal is produced by electrolysis. Apreferred cell for production of aluminum by electrolysis of aluminacomprises a pot defining a chamber containing a molten electrolyte, acathode, at least one anode contacting the electrolyte, and a solidcrust above the electrolyte. The crust comprises solidified electrolyteand alumina, and may build up to a thickness of several inches.

The molten electrolyte comprises sodium fluoride and aluminum fluoridein a weight ratio of about 0.7-1.2, together with lesser amounts ofmagnesium fluoride and calcium fluoride. The molten electrolyte has atemperature of at least about 900° C., more preferably about 900-1050°C. The electrolyte is preferably maintained at a temperature of about960-980° C. As reduction takes place, a pad of molten aluminum settleson the cell bottom, above the cathode.

In order to tap molten aluminum from the cell periodically the crust isbroken, leaving a hole through which heat is lost from the electrolyte.Cell voltage must then be increased to compensate for the heat loss,resulting in increased electric power consumption.

Heat loss through the crust is reduced by repairing the crust hole. Thehole is covered with a receptacle containing a flowable mass of solidparticles. The receptacle is preferably closed.

The receptacle comprises a polymeric material. As used herein, the term“polymeric material” includes cellulosic materials, polyolefins such aspolyethylene and polypropylene, polyesters such as polyethyleneterephthalate, and polyamides.

A preferred receptacle comprises a cellulosic material, more preferablyone or more of paper, polymer-impregnated paper, and cardboard. Thecellulosic material is derived from wood, reclaimed paper, abaca, jute,or a mixture thereof. A paper bag is particularly preferred.

The paper bag has a plurality of walls, with at least one wallcomprising at least two layers of paper. The receptacle and the solidparticles together weigh about 11-33 lb (5.0-15.1 kg), preferably about15-20 lb. (6.8-9.1 kg). The receptacle and the solid particles togetherprovide a covering having a depth of a few inches to several inches overthe crust hole, preferably about 2-6 inches (5-15 cm), and about 3inches (7.5 cm) in a preferred embodiment. Solid particles in the paperbag comprise a flowable mass so that the filled bag is sufficientlyflexible to follow the upper contour of the crust.

The electrolytic cell of the invention preferably produces aluminum byelectrolyzing alumina dissolved in a molten salt bath electrolyte. Othermetals produced by similar electrolytic processes include magnesium,zinc, lithium, and lead.

When the cell produces aluminum by electrolysis of alumina, the solidparticles preferably comprise one or more aluminum compounds. Somesuitable aluminum compounds includes alumina, aluminum fluoride,cryolite, and mixtures thereof in various proportions. One suitable formof alumina is smelting grade alumina (SGA), typically having an LOI ofless than 1 wt. %, 99 m²/g surface area, average pore (SGA), typicallyhaving an LOI of less than 1 wt. %, 99 m²/g surface area, average porevolume 0.224 cm³/g, average pore size about 92 microns, and −100+325mesh size. SGA is commonly used as a feedstock for aluminum smelters.Another suitable form of alumina is electrostatic precipitator dust (ESPdust) obtained from pollution control devices in aluminum smeltingplants and in aluminum refineries.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an electrolytic cell ofthe invention.

FIG. 2 is a schematic, fragmentary cross-sectional view of theelectrolytic cell of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

There is shown in FIG. 1 an electrolytic cell 10 for aluminumproduction, having carbon anodes 12 suspended from a movable bridge 14.The anodes 12 are situated above a pot or cell 16 lined with a layer ofinsulating material 18 upon which solid carbon cathode blocks 20 arepositioned. The cathode blocks 20 are connected in an electrical circuitwith an external bus 22 via steel collector bars 24 passing through thecathode blocks 20.

A bath 26 of molten cryolite containing dissolved alumina is maintainedat approximately 950-960° C. within the pot 16 and as reduction takesplace a pad 28 of molten aluminum settles over the cathode blocks 20. Alayer of crust 30 forms above the bath 26, surrounding the carbon anodes12. The crust 30 generally has a thickness of several inches.

The movable bridge 14 is vertically adjustable to enable the carbonanodes to be elevated or lowered relative to the height of the bath 26.An overhead hopper 34 supported between the carbon anodes 12 is filledwith alumina ore. Alumina ore from the hopper is 34 periodically addedto the bath 26 as needed through a feeder mechanism 36. The feedermechanism 36 includes a downwardly projecting steel rod 38 supporting aceramic plugger foot 40. When alumina ore is added to the bath 26, thesteel rod 38 and plugger foot 40 are thrust downwardly to punch a holethrough the crust 30. An overhead conveyor 42 supplies alumina ore tothe hopper 34 as needed.

Tapping molten aluminum from the metal pad 28 requires breaking thecrust 30 to insert a vacuun tap (not shown). In a typical Hall-Heroultelectrolytic cell, molten aluminum is tapped approximately every 24hours. After the tap is removed, a hole remains in the crust 30 abovethe molten electrolyte 26. Holes left over from molten metal tappingtypically have dimensions of about 12 in×12 in (30 cm×30 cm).

In accordance with the present invention, a hole 50 in the crust 30 isrepaired by placing a paper bag 55 directly above the hole 50 as shownin FIG. 2. The bag 55 is double walled on all sides and is filled withapproximately 20 lb (9.1 kg) of a mixture of smelting grade alumina andcrushed molten salt bath. A mixture of about 10 lb (4.5 kg) SGA and 10lb (4.5 kg) crushed molten salt bath is quite suitable.

Alumina and bath particles in the bag 55 are sintered into a porous massby heat from the molten electrolyte 26. The bag 55 remains intact for asufficient time to prevent the solid particles from scattering.Eventually, the bag 55 is oxidized, some of the solid particles mergewith the crust 30 and some of the solid particles drop down into themolten electrolyte 26. Containing the particles in the paper bag 55eventually restores the crust 30 to an unbroken, unitary mass withoutimbalancing alumina content in the electrolyte 26.

Having described the presently preferred embodiments, it is to beunderstood that the invention may be otherwise embodied within thespirit and scope of the appended claims.

What is claimed is:
 1. In an electrolytic cell wherein a metal isproduced by electrolysis, said cell comprising a molten electrolyte atan elevated temperature and above said electrolyte a crust defining ahole through which heat is lost from said electrolyte, the improvementin the operation of said cell comprising repairing said hole by coveringit with a receptacle containing solid particles comprising alumina. 2.The improvement of claim 1, wherein said receptacle comprises apolymeric material.
 3. The improvement of claim 2, wherein saidpolymeric material comprises a cellulosic material.
 4. The improvementof claim 3, wherein said receptacle comprises a cellulosic materialselected from the group consisting of paper, polymer-impregnated paper,and cardboard.
 5. The improvement of claim 3, wherein said cellulosicmaterial is derived from wood, reclaimed paper, abaca, jute, or amixture thereof.
 6. The improvement of claim 1, wherein said receptacleis closed.
 7. The improvement of claim 1, wherein said receptaclecomprises a paper bag.
 8. The improvement of claim 7, wherein said paperbag has a plurality of walls and at least one of said walls comprises atleast two layers of paper.
 9. The improvement of claim 1, wherein saidreceptacle and said solid particles together weigh about 11-33 lb.(5.0-15.1 kg).
 10. The improvement of claim 1, wherein said receptacleand said solid particles weigh about 15-20 lb. (6.8-9.1 kg).
 11. Theimprovement of claim 1, wherein said receptacle and said solid particlescomprise a covering having a depth of about 2-6 in. (5-15 cm) over saidhole.
 12. The improvement of claim 1, wherein said molten electrolytehas a temperature of at least about 900° C. and heat from saidelectrolyte sinters said solid particles.
 13. The improvement of claim1, wherein said molten electrolyte has a temperature of about 900-1050°C.
 14. The improvement of claim 1, wherein said electrolytic cellproduces aluminum by electrolysis of alumina, and said solid particlesfurther comprise an aluminum compound selected from the group consistingof aluminum fluoride, cryolite, and mixtures thereof.
 15. Theimprovement of claim 14, wherein said aluminum compound is selected fromthe group consisting of alumina, aluminum fluoride, cryolite, andmixtures thereof.
 16. The improvement of claim 1, wherein said solidparticles comprise ESP dust.
 17. The improvement of claim 1, whereinsaid crust supports said receptacle above said electrolyte.
 18. Theimprovement of claim 17, wherein said receptacle is spaced from saidelectrolyte.
 19. An electrolytic cell for producing a metal byelectrolysis, said cell comprising a chamber containing a moltenelectrolyte at an elevated temperature, above said electrolyte a crustdefining a hole, and a covering for said hole comprising a receptaclecontaining solid particles, said crust supporting said receptacle upwardof and spaced from said electrolyte, thereby reducing heat loss fromsaid electrolyte through said hole.
 20. The cell of claim 19, whereinsaid receptacle comprises a cellulosic material selected from the groupconsisting of paper, polymer-impregnated paper, and cardboard.
 21. Thecell of claim 19, wherein said cell produces aluminum by electrolysis ofalumina dissolved in said molten electrolyte, and said moltenelectrolyte has a temperature of at least about 900° C.
 22. The cell ofclaim 19, wherein said solid particles comprise an aluminum compoundselected from the group consisting of alumina, aluminum fluoride,cryolite, and mixtures thereof.
 23. The cell of claim 19, wherein saidsolid particles comprise alumina.
 24. The cell of claim 23, wherein saidsolid particles further comprise aluminum fluoride or cryolite.